It is well known that the distance traveled by a golf ball following impact with a golf club driver is directly related to clubhead speed. Various methods have been devised to reduce the overall aerodynamic drag of golf clubs in order to increase clubhead speed. All share a common denominator which is making the components of a golf club more aerodynamically shaped.
With regard to the golf club shaft, the drag coefficient of a shaft with a circular cross-section is as much as 25 times greater than that for a shaft with a streamlined cross-section, for equal cross-sectional heights. As early as 1921, attempts have been made to make the shaft more aerodynamic, as evident in prior U.S. Pat. Nos. 1,396,470 to Taylor, and 1,528,017 to Gammeter.
Despite these and other efforts, all current United States Golf Association (USGA) legal shafts on the market today have circular cross-sections, largely due to the constraints of USGA regulation 4-1b which specifies that at any point along its length the shaft shall:
(i) Bend in such a way that the deflection is the same regardless of how the shaft is rotated about its longitudinal axis. PA1 (ii) Twist the same amount in both directions. PA1 (i) For clubs other than putters the grip must be circular in cross-section . . . PA1 1. A sheath which offers high compressibility, and thus does not alter the bending characteristics of the circular shaft, probably embodies low resiliency. This sheath would be of low durability. PA1 2. In order to use an industry standard grip, the shaft's exterior shape must transform from an elliptical shape to a circular shape. This shape transformation requires a non-abrupt transformation of the truss structure to preserve equal bending characteristics of the shaft. The shape of this transformed truss structure is not disclosed, and is envisioned to be virtually impossible to manufacture. PA1 3. Varying the wall thickness at a given cross-section of the shaft is considered to be difficult in a conventional filament winding process or steel tube drawing process.
Additionally, regulation 4-1c specifies:
A shaft employing a circular cross-sectional shape is symmetric about any axis orthogonal to the shaft's longitudinal axis, and thus offers equal bending and twisting properties regardless of orientation around the longitudinal axis. Such a shaft is readily manufactured in various fashions, including drawn metallic tubing and filament wound composites. Moreover, symmetrical shapes such as squares might be employed if the drag coefficient was not even greater than that of a circular shape. The limiting factors which have in the past prevented the introduction of a shaft with a streamlined cross-section, satisfying USGA regulations, are materials and manufacturing methods. Recently however, advances in these areas and in the area of computer aided design/manufacturing/engineering (CAD/CAM/CAE) now allow for such a shape.
It can be readily imagined by those skilled in the art a shaft whose external shape is more aerodynamic than that of a shaft with a circular cross-section. More difficult to imagine is the internal structure which will allow for this shape. Prior U.S. Pat. No. 5,335,908 to Bamber (1994) broadly suggests an elliptical shape to reduce drag. This patent proposes to fixate a compressible and resilient sheath around a circular shaft, install a "truss structure" within the interior of the shaft, or vary the wall thickness of the shaft. The disadvantages with these methods which limit practicality are:
U.S. Pat. Nos. 2,018,723 to Hutchison (1935), and 5,632,692 to Lebovici (1997), suggest golf shafts with generally triangular cross-sectional shapes orientated to present the tapered leading edge to the flow of air, with the object of reducing drag. These proposals would offer minimal reductions in shaft drag, and may increase drag due to separated airflow aft of the maximum cross-sectional height. In fact, for Reynolds numbers of up to two million, a two dimensional triangular shape has a drag coefficient of 1.3, as compared to a drag coefficient of 1.0 for a circular shape (McCormick).
McCormick states that a 2-D streamlined shape with a fineness ratio of 3.0 produces a minimum profile drag coefficient, where fineness ratio is the body width aligned with the flow of air divided by the body height. Thus, neglecting structural considerations, a streamlined golf club shaft should have a width to height ratio on the order of 3.0 to minimize drag. The previously mentioned prior art under-estimate the importance of an after-body on the reduction of drag, and suggest shapes which abruptly end after the point of maximum cross-sectional height.
Although the referenced prior art address the goal of reducing shaft drag, as of this invention's filing a streamlined shaft which meets USGA regulations is not commercially available. There still exists a need for an aerodynamic golf shaft which promotes an increased clubhead speed, whose exterior is durable in nature, and which meets the USGA regulations with regard to bending and twisting.